Direction-of-arrival (DoA) estimation of a signal emitted by a source or signal source of unknown location involves in the state of art an receive antenna array. Relying on the delay the signal is received amongst the antennas, a conclusion on the direction of the source can be drawn.
Not only the location of the signal source, but usually also the source orientation concerning the emission of signals is unknown. Thus, the polarization of the source signal can be arbitrarily inclined with respect to the receive array. Therefore, receive arrays of linearly polarized elements are usually deployed, because the phase difference amongst the elements is preserved for varying source inclination. This, however, can cause polarization mismatches, possibly leading to weak receive signals, whose direction of arrival cannot be accurately estimated.
Circularly polarized receive antennas do not suffer from polarization losses caused by inclined sources, assuming a linearly polarized source signal. Yet because of the limited axial-ratio beam width, which is displayed by a circularly polarized antenna, the phase difference amongst the elements varies with varying source inclination (see FIG. 1).
The scientific community has been investigating various antenna arrays and concepts to increase the accuracy of direction finding (DF).
This includes approaches using multibeam antennas (MBA) [2, 3]. Typically these antennas are linearly polarized and so are the signal sources. Thus, even if the signal source is arbitrarily oriented, the phase of the received signals stays mostly constant for a given impinging angle [4]. This allows for estimating of the DoA without considering source inclination, at the expense of a reduced estimation performance caused by polarization mismatches.
To achieve a DoA estimation performance being independent of the source orientation, a circularly polarized direction finding antenna is advantageous. However, for such an antenna, the received signal phases vary with changing signal source inclination, due to the limited axial ratio beamwidth.
There is a variety of methods for direction finding such as subspace-based methods, e.g. MUSIC [5]. Amongst all methods, the accuracy strongly depends on the set of steering vectors used and how accurate those represent the antenna characteristics. This includes polarization and the inclination of the source with respect to the antenna. As abovementioned, polarization mismatch causes inaccurate DoA estimates.
To cope with polarization mismatch, receive antenna arrays with dual polarization can be used. These arrays have different steering vectors for different source inclination and are therefore referred as polarization sensitive arrays [6]. Each element provides two signals, one for the first polarization and one for the second polarization. A set of two orthogonal polarization is typically used, e.g., linear vertical and linear horizontal polarization or right handed circular and left handed circular polarization (RHCP & LHCP). This, however, doubles the implementation effort, as twice as many signal branches are needed compared to an array supporting one polarization. Hence, an array supporting a single polarization only is advantageous in practice.